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User blog:Samuli.seppanen/Elastic versus stiff arms
Since the last blog posting I've had several shooting sessions. The first two related sessions from a couple of weeks ago were done with the so-called elliptically tillered arms I talked about earlier. During those sessions I shot the weapon for about one hundred times, counting all the control shots and unrecorded shots between sessions. Here are the results for each bolt weight: I attempted to increase washer rotation to 150 degrees, but I was then unable to cock the weapon with stomach pressure alone. I then took a 6mm thick spring steel rod, rested it against the feet of the pittarion and used that to assist the draw. This enabled me to cock the weapon at 150 degree rotation. This increased the power somewhat, but on the second cocking one of the arms pretty much exploded at the hoop when approaching full draw. Now, as can be seen from above, the above results were not particularly impressive. In my record shooting session I was consistently able to reach 114 joules with 49,70 gram bolt. On top of that I was able to squeeze out the current record of 124 joules, and that session ended with the bowstring slipping from under the claw. So the arms were not the weak point back then. The latest two sessions were outdoor sessions with fletched bolts, where I used the old semi-elliptically tillered arms that I had used in the record shooting session. The only difference was replacement of the double round hoops with a single, larger hoop. For these two sessions I don't have as much data as for the earlier ones: Cocking by stomach pressure alone was still very feasible and I was able to easily reach the record numbers I had really struggled for using the elliptically tillered arms of the earlier shooting sessions. While the weight of the elliptically tillered arms was somewhat higher and their weight placement less optimal, that does not explain the rather large difference - otherwise the heaviest 87,62 gram test bolt should have showed better results compared to the lighter bolts. In reality the difference was rather minor, as can be seen from the data above. The latest session ended up in one of the cones snapping right next to the hoop, and the other was queuing to be next. I believe this has to do with elasticity of the arms: the elliptically tillered arms were much stiffer than the old, semi-elliptically tillered arms. The former did not bend visibly even at full draw, whereas the latter bent very clearly. This is something that Dr. Burkhard Meissner and his research team had found out in their tests, and which they kindly shared with me via email in late 2016, before their official research article came out. So, in order for the elliptical tiller to work properly the arms would have to be made thinner, so that they are actively bending. I also have to concur with another finding Dr. Burkhard Meissner shared with me in the same email thread. His team found out that wood alone cannot stand the compressive forces of the spring cord. I was aware of this problem, but did not still realize its full importance back then. Now that all the other components work perfectly and are strong enough the last two sets of arms snapped right next to the hoop like a toothpick. I believe the main reason is that the spring cord really crushes the wood in high-power shooting. The crushing action does not happen instantly, but over time, which allows several dozens of shots before the wood is crushed thin enough to create a weak point, which gets overstressed and finally gives in. On the back of the arm the bar seems to adequately prevent the crushing action. So, my next step is to create a new pair of elastic arms with added steel protection at the base of the cone at the belly-side. I believe this finally allows going to 130 joules or more with nylon. That is, I believe, the final technical issue that remains to be resolved. Then I can start testing with specially treated animal fibers mentioned by Dr. Burkhard Meissner in his 2017 article. Based on the data from previous shooting sessions the optimal bolt weight seems to be around 30 grams. Increasing bolt weight to 45-50 grams does not increase energy that much, and reducing bolt weight to 20-25 grams results in rather large drop in energy. When the new arms with belly-side metal protection are ready, I believe one can reach slightly more than 80 m/s with 30 gram bolts, or about 90 m/s with 20 gram bolts. What I also found out is that using a simple steel bar resting against the feet of the pittarion as a handle is more than adequate. This helps squeeze the final 10-20 joules out of the weapon, while retaining the auto-locking property of the handle. Category:Blog posts Category:Backup Category:Statistics Category:Practical Category:Theoretical Category:Arms